Method and device for hitless tunable optical filtering

ABSTRACT

The method for filtering an optical signal comprising a plurality of channels lying on a grid of optical frequencies equally spaced by a frequency spacing and occupying an optical bandwidth, comprises: a) operating an optical filter comprising a plurality of resonators, wherein a first resonator of the plurality is optically coupled to the optical signal and the remaining resonators are optically coupled in series to the first resonator, so that a respective resonance of each one of the plurality of resonators falls within a first frequency band having bandwidth less than or equal to 15 GHz; b) operating the optical filter so as to obtain a separation between said respective resonance of at least one resonator with respect to said respective resonance of at least another different resonator, the separation being greater than or equal to 25 GHz; c) operating the optical filter so that said respective resonance of each one of the plurality of resonators falls within a second frequency band, different from the first frequency band, having bandwidth less than or equal to 15 GHz, wherein during the procedure from step a) to step c), at least one among said respective resonance of the at least one resonator and said respective resonance of the at least another different resonator is moved also outside a frequency region spanning between, and including, the first and the second frequency band. A corresponding device for filtering an optical signal is disclosed.

FIELD OF THE INVENTION

The present invention relates to the field of optical communicationsystems including hitless tunable optical filtering functionality, suchas hitless tunable optical add and/or drop functionality.

BACKGROUND OF THE INVENTION

A common technique to increase the transmission capacity of todayoptical communication systems is wavelength division multiplexing (WDM),wherein a plurality of optical channels, each having a respectiveoptical frequency (and correspondingly respective optical wavelength),are multiplexed together in a single optical medium, such as for examplean optical fiber. The optical frequencies allocated for the WDM channelsare typically arranged in a grid having an equal spacing between twoadjacent frequencies. In dense WDM (DWDM), wherein the WDM channels maybe closely spaced, the frequency spacing is typically equal to about 100GHz (corresponding to a wavelength spacing of about 0.8 nm in the nearinfrared band—roughly between 1 μm to 2 μm) or about 50 GHz (about 0.4nm in wavelength). Other used WDM channel separations are 200 GHz, 33.3GHz and 25 GHz. Typically, the set of allocated optical frequenciesoccupies an optical bandwidth of about 4 THz, which gives room for theuse of up to 40 or 41 WDM channels having 100 GHz spacing. The device ofthe present invention is suitable for a WDM optical bandwidth of atleast about 1 THz, preferably at least about 2 THz, typically placedaround 1550 nm.

Optical networking is expected to be widely used in perspective opticalcommunication field. The term ‘optical network’ is commonly referred toan optical system including a plurality of point-to-point orpoint-to-multipoint (e.g., metro-ring) optical systems opticallyinterconnected through nodes. In all-optical transparent networks few orno conversions of the optical signal into electrical signal, and thenagain in optical signal, occur along the whole path from a departurelocation to a destination location. This is accomplished by placing atthe nodes of the optical networks electro-optical or optical deviceswhich are apt to process the optical signal in the optical domain, withlimited or no need for electrical conversion. Examples of such devicesare optical add and/or drop multiplexers (OADM), branching units,optical routers, optical switches, optical regenerators (re-shapersand/or re-timers) and the like. Accordingly, the term ‘opticalfiltering’ or ‘optical processing’, for the purpose of the presentdescription is used to indicate any optical transformation given to anoptical radiation, such as extracting a channel or a power portion ofsaid channel from a set of WDM channels (‘dropping’), inserting achannel or a power portion of said channel into a WDM signal (‘adding’),routing or switching a channel or its power portion on a dynamicallyselectable optical route, optical signal reshaping, retiming or acombination thereof. In addition, optical systems, and at a greaterextent optical networks, make use of optical amplifiers in order tocompensate the power losses due to fiber attenuation or to insertionlosses of the optical devices along the path, avoiding the use of anyconversion of the optical signal into the electrical domain even forlong traveling distances and/or many optical devices along the path. Incase of DWDM wavelengths, all channels are typically optically amplifiedtogether, e.g. within a bandwidth of about 32 nm around 1550 nm.

In optical systems, and at a greater extent in optical networks, aproblem exists of filtering one or more optical channels at the nodeswhile minimizing the loss and/or the distortion of the filtered opticalchannel(s), as well the loss and/or the distortion of the opticalchannels transmitted through the node ideally without being processed(hereinafter referred to as ‘thru’ channels). Advantageously, theoptical processing node should be able to simultaneously process morethan one channel, each one arbitrarily selectable independently from theother processed channels. Ideally up to all the channels may besimultaneously selectable to be processed, but in practice a numberbetween 2 and 16, preferably between 4 and 8, is considered to besufficient for the purpose.

It is desirable that the optical processing node is tunable orreconfigurable, i.e., it can change dynamically the subset of channelson which it operates. In order to be suitable to arbitrarily select thechannel to be processed within the whole WDM optical bandwidth, thetuning range of the whole optical processing node should be at leastequal to said optical bandwidth.

It is also preferred that while the processing node “moves” from aninitial channel (A) to a destination channel (B), the channels differentfrom A and B remain unaffected by the tuning operation. In this case thecomponent is defined as ‘hitless’. In particular, the channels placedbetween the initially processed channel and the final channel aftertuning should not be subject to an additional impairment penalty, called‘hit’, by the tuning operation. The hit may include a loss penaltyand/or an optical distortion such as phase distortion and/or chromaticdispersion.

For example, optical communication networks need provisions forpartially altering the traffic at each node by adding and/or droppingone or several independent channels out of the total number. Typically,an OADM node removes from a WDM signal a subset of the transmittedchannels (each corresponding to one frequency/wavelength), and adds thesame subset with a new information content, said subset beingdynamically selectable.

There are several additional concerns. The tunable optical processingnode should not act as a narrow band filter for the unprocessedchannels, since concatenation of such nodes would excessively narrow thechannel pass bands. The tunable optical processing node should also beultra-compact and should have low transmission loss and low cost, sincethese important factors ultimately determine which technology isselected.

In article “Non-blocking wavelength channel switch using TO effect ofdouble series coupled microring resonator”, S. Yamagata et al., El.Lett. 12 May 2005, Vol. 41, No. 10, it is demonstrated a non-blockingtunable filter using the thermo-optic (TO) effect of a double seriescoupled polymer microring resonator by controlling individual resonantwavelengths.

In article “Fast and stable wavelength-selective switch usingdouble-series coupled dielectric microring resonator”, Y. Goebuchi etal., IEEE Phot. Tech. Lett., Vol. 18, No. 3, Feb. 1, 2006, it isdemonstrated a hitless tunable add-drop filter using the thermo-opticeffect of double series coupled dielectric microring resonator.

SUMMARY OF THE INVENTION

The Applicant has found that there is a need for an opticalcommunication system having tunable optical processing functionalitywhich leaves unaltered, or at least reduces the alteration of, the thruchannels during tuning, i.e. it should be ideally hitless. In addition,the optical processing node should preferably be low-loss, low-cost,fast tunable and/or broadband.

The Applicant has noted that the filter devices described in the abovecited articles are not optimally designed and/or operated for changing(tuning) the filtered optical channel from an initial channel to a finalone while keeping at zero or low level the power and/or dispersion hiton the thru channels (placed in between the initial and final channeland/or outside the spectral region spanning from the initial to thefinal channel) during the entire tuning procedure.

The Applicant has found a method and a system for optical transmissionprovided with tunable optical processing functionality which can solveone or more of the problems stated above. The solution of the presentinvention is simple, feasible and low cost.

In an aspect of the present invention, a method for filtering an opticalsignal is provided.

The applicant believes that, during tuning of an optical filtercomprising a plurality of resonators, the fact that a resonance of atleast one resonator is moved outside the frequency region strictlynecessary for going from the initial channel to the final one, allowsachieving ‘hitless’ or low hit tuning of the overall filter.Advantageous embodiments of this method are provided.

According to another aspect of the present invention, an optical devicefor filtering an optical signal is provided. Advantageous embodiments ofthis device are provided.

In an embodiment of this aspect, said at least one among said respectiveresonance of said at least one resonator and said respective resonanceof said at least another different resonator is moved outside saidfrequency region either during step b) or during step c).

In an embodiment of this aspect, said at least one among said respectiveresonance of said at least one resonator and said respective resonanceof said at least another different resonator belongs to a resonatordifferent from said first resonator.

In an embodiment of this aspect, both said respective resonance of saidat least one resonator and said respective resonance of said at leastanother different resonator are also moved outside the frequency regioncomprised between, and including, the first and the second frequencyband.

In an embodiment of this aspect, one among said respective resonance ofsaid at least one resonator and said respective resonance of said atleast another different resonator is moved outside said frequency regioneither during step b) or during step c) and the other among saidrespective resonance of said at least one resonator and said respectiveresonance of said at least another different resonator is moved outsidesaid frequency region respectively either during step c) or during stepb).

In an embodiment of this aspect, the control system is furtherconfigured to perform, after step b) and before step c), the step of: d)tuning all the resonators of the optical filter so as to move allrespective resonances of the resonators by a frequency interval greaterthan said frequency spacing while maintaining a distance between saidresonance of said at least one resonator with respect to said respectiveresonance of said at least another different resonator not less thansaid separation.

In a preferred embodiment of the latter embodiment, in step d) all theresonators are tuned substantially in unison so as to equally andcontemporarily move all said respective resonances of the resonators.

In an embodiment of this aspect, the control system is configured totune at least one resonator of the optical filter differentially fromthe remaining resonators.

In an embodiment of this aspect, all the resonators of the opticalfilter have the same free spectral range:

In an embodiment of this aspect, said separation is greater than orequal to 150 GHz.

In an embodiment of this aspect, said separation is less than or equalto 1000 GHz.

In an embodiment of this aspect, in step b) the optical filter isoperated so as to obtain said separation between each resonance of saidat least one resonator falling within said optical bandwidth withrespect to the respective closest resonance of said at least anotherdifferent resonator.

In an embodiment of this aspect, said plurality of resonators of theoptical filter comprises two and no more than two resonators.

According to a further aspect of the present invention, an opticalcommunication system comprises a transmitter, a receiver, an opticalline optically connecting the transmitter and the receiver and anoptical device according to the above and coupled along the opticalline.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be made clearby the following detailed description of an embodiment thereof, providedmerely by way of non-limitative example, description that will beconducted making reference to the annexed drawings, wherein:

FIG. 1 schematically shows in terms of functional blocks an exemplaryoptical communication system architecture according to the presentinvention;

FIG. 2 is a schematic diagram showing in terms of functional blocks anembodiment of the device for tunable optical filtering according to thepresent invention;

FIG. 3 is a schematic diagram showing in terms of functional blocks afurther embodiment of the device for tunable optical filtering accordingto the present invention;

FIG. 4 shows the principal steps of several comparative examples of amethod for tuning an optical filter.

FIGS. 5A, B, C and D show the effects in terms of optical powerresponses of the methods of FIG. 4.

FIGS. 6A, B and C show the effects in terms of dispersion responses ofthe methods of FIG. 4.

FIGS. 7A, 7B and 7C show several embodiments of a method for tuning anoptical filter in accordance to the present invention.

FIG. 8 shows the effects in terms of optical power of the methods ofFIGS. 7A, B and C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

FIG. 1 shows an optical communication system architecture according to apossible embodiment of the present invention.

The optical communication system 100 comprises at least a transmitter110, a receiver 120 and an optical line 130 which optically connects thetransmitter and the receiver. The transmitter 110 is an opto-electronicdevice apt to emit an optical signal carrying information. It typicallycomprises at least an optical source (e.g., a laser) apt to emit anoptical radiation and at least a modulator apt to encode informationonto the optical radiation. Preferably, the transmitter 110 is a WDMtransmitter (e.g., a DWDM transmitter) and the optical signal maycomprise a plurality of optical channels (each carryingmodulation-encoded information) having respective optical frequenciesequally spaced by a given frequency spacing and occupying an opticalbandwidth. Preferably, said optical signal lies in the near-infraredwavelength range, e.g. from 900 nm to 1700 nm. Preferably said opticalbandwidth is at least 1 THz, more preferably it is at least 2 THz, stillmore preferably it is at least 3 THz, such as for example equal to about4 THz (e.g. from about 1530 to about 1560 nm, called C-band). Thereceiver 120 is a corresponding opto-electronic device apt to receivethe optical signal emitted by the transmitter and to decode the carriedinformation. The optical line 130 may be formed by a plurality ofsections of optical transmission media, such as for example opticalfiber sections, preferably cabled. Between two adjacent sections ofoptical fiber, an optical or opto-electronic device is typically placed,such as for example a fiber splice or a connector, a jumper, a planarlightguide circuit, a variable optical attenuator or the like.

For adding flexibility to the system 100 and improving systemfunctionality, one or a plurality of optical, electronic oropto-electronic devices may be placed along the line 130. In FIG. 1 aplurality of optical amplifiers 140 are exemplarily shown, which may beline-amplifiers, optical boosters or pre-amplifiers.

According to the present invention, the optical system 100 comprises atleast one optical processing node (OPN) 150 optically coupled to theoptical line 130 and apt to filter or route or add or drop orregenerate, fully or partially, at least one optical channel of the WDMoptical signal propagating through the optical line 130. The OPN ispreferably dynamically tunable or reconfigurable. In the particular casewherein the optical processing node 150 is an optical add and/or dropnode 150, as shown in FIG. 1, i.e., a node adapted to route or switch oradd and/or drop the optical signal, the routed or switched or dropped oradded channel(s) may be received or transmitted by further receiver(s)152 or transmitter(s) 154, respectively, which may be co-located withthe OPN node or at a distance thereof. The optical system or network 100may advantageously comprise a plurality of optical processing nodes. InFIG. 1 a further optical processing node 150′ is exemplarily shown,together with its respective optional transmitting and receiving devices152′ and 154′.

An optical system 100 having optical add and/or drop nodes 150, as shownin FIG. 1, is commonly referred to as an optical network and it ischaracterized by having a plurality of possible optical paths for theoptical signals propagating through it. As exemplarily shown in FIG. 1,a number of six optical paths are in principle possible, whichcorresponds to all possible choices of the transmitter-receiver pairs inFIG. 1 (excluding the pairs belonging to the same node).

FIG. 2 shows a schematic diagram of an optical device 200 in accordancewith an embodiment of the present invention. The optical device 200 maybe comprised within the optical processing node 150 of FIG. 1.

The design scheme of the optical device 200 according to the presentinvention comprises an optical filter 250 comprising an input port 257and an output port 258. Throughout the present description, the terms‘input’ and ‘output’ are used with reference to a conventional directionof propagation of the optical radiation (in FIG. 2 exemplarily from leftto right and from top to bottom, as indicated by the solid arrows), but,when in operation, the optical radiation may propagate in the oppositedirection. The optical filter 250 is adapted to receive an opticalsignal comprising a plurality of channels lying on a grid of opticalfrequencies equally spaced by a given frequency spacing and occupying anoptical bandwidth via the input port 257 and to output a transformedoptical signal via the output port 258 according to optical transferfunctions (such as phase and power transfer functions). The opticalfilter 250 may be any optical device apt to give an opticaltransformation to the input optical signal, being its optical transferfunctions wavelength-dependent in the wavelength band of interest. Inthe present description, any physical quantity which substantiallychanges within the WDM optical wavelength band of interest (e.g. 30 nmaround 1550 nm) is referred to as being ‘wavelength-dependent’.

Comprised within the optical filter 250, a first optical path 230, inthe form of, e.g. an optical waveguide such as a planar lightguidecircuit (PLC) waveguide, optically connects the input port to the outputport.

According to the present invention the optical filter 250 comprises aplurality of resonant cavities (or resonators) 252, 254, 255, such asBragg gratings or microcavities such as linear cavities, microrings,racetracks, photonic band gap cavities and the like. In a preferredconfiguration, the resonant optical filter 250 comprises microring orracetrack resonators. The plurality of resonators comprises a firstoptical resonator 252 optically coupled adjacently to the first opticalpath 230 and one or more further resonators 254, 255 coupled in seriesto said first resonator 252. In the drawings, the symbol 255 consistingin three dots vertically aligned represents an arbitrary number,including zero, of resonators. Preferably the series-coupled resonators252, 254, 255 comprised within the optical filter 250 are less than orequal to four, more preferably they are two or three.

In general, a single resonant optical cavity has associated ‘resonantwavelengths’ (and corresponding ‘resonant frequencies’), defined asthose wavelengths which fit an integer number of times on the cavitylength of the resonant optical cavity. The integer number defines theorder of the resonance. For example a Bragg grating comprises aplurality of coupled resonant cavities. The distance between twoadjacent resonant frequencies/wavelengths is referred to as the freespectral range (FSR) of the single resonator.

The transfer functions (e.g. phase, dispersion or power) of the aboveresonant optical filter 250 are typically characterized by strongwavelength dependence at and in the proximity of a resonant wavelengthof one or more of its individual resonators, depending on thedistribution of the resonances of the constituting individual resonatorsand on their reciprocal position in the frequency domain. The aboveperturbations of the overall transfer function are typically equallyspaced in frequency and the distance between two adjacent perturbationsof the optical filter 250 is referred to as the (overall) ‘free spectralrange’ of the resonant optical filter 250. In the advantageous casewherein all the resonators comprised within the optical filter have thesame FSR and their resonances are all aligned, the overall FSR of theoptical filter coincides with the FSR of the single resonators and theoverall resonances of the optical filter coincides with the resonancesof the (aligned) resonators.

The overall FSR of the optical filter 250 may be greater or smaller thanthe WDM optical bandwidth.

In a preferred embodiment the optical filter 250 is an optical addand/or drop filter (OADF) wherein the two or more resonators 252, 254,255 are optically coupled in series between the first optical path 230and a further ‘drop’ waveguide 256. A further output optical port 260(‘drop port’), optically coupled to the drop waveguide has the functionof dropping, fully or partially, at least an optical channel within theinput optical signal. In other words, the power transfer function at thedrop port 260 is typically characterized by high transmission peaksequally spaced in frequency by a quantity equal to the overall FSR ofthe optical filter. In an embodiment, the OADF 250 has a further inputoptical port 259 (‘add port’) which is apt to receive an opticalradiation (dashed arrow) to be added to the thru optical signal at theoutput port 258. It is noted that in the absence of the further dropwaveguide, the optical filter 250 may act as an all-pass filter. In FIG.2 the positions of the drop and add port are those determined by an evennumber of resonators. In case of an odd number of resonators, thepositions of the two ports may be switched.

When an optical drop filter 250 having an even number of resonators isin operation, the optical channels input into the input port 257 andhaving optical frequencies which match the (aligned) resonances of theresonators 252, 254, 255 are output into the drop output port 260coupled to the second optical path 256, and they physically travelacross the resonators 252, 254, 255, as indicated by the down-arrow nearthe microrings.

According to the present invention, the optical filter 250 is a tunableoptical filter, i.e. it is apt to select an arbitrary optical channel tobe filtered. This functionality may be accomplished with any techniqueknown in the art, such as for example exploiting the thermo-optic, theelectro-optic, the magneto-optic, the acousto-optic and the elasto-opticeffect or by stress or MEMS actuating. In particular, at least oneresonator comprised within the optical filter 250 is individuallytunable differentially with respect to at least another resonator of theremaining resonators of the optical filter 250, i.e. it may be tunedwith a certain degree of freedom from the tuning of the at least anotherresonator. In the present description, the expression ‘tuning anindividual resonator’ means moving the resonances of the resonator inthe frequency spectrum, e.g. actuating the resonator by exploiting aphysical effect. Typically, the ensemble of resonances of an individualresonator moves substantially rigidly (i.e. maintaining unchanged theresonance distribution and spacing) while the resonator is being tuned.

The optical device 200 further comprises a control system 270, 272, 251,253 operatively connected to the resonators of the optical filter 250 soas to be able to selectively tune them in accordance to the presentinvention.

For example, the control system may comprise a control device 270operatively coupled, by way for example of connecting lines 272, to atleast two actuators 251, 253, which in turn are operatively coupled tothe plurality of resonators of the optical filter 250. The controldevice 270 typically includes a processor (e.g. a microprocessor)configured for implementing the methods of tunable filtering inaccordance with the present invention. The control system may alsoinclude drivers (not shown) suitable to drive the actuators 251 and 253.

In one exemplary embodiment, as the one shown in FIG. 2, each individualresonator may be tuned, within a tuning range, substantiallyindependently from the remaining resonators, by way for example of arespective actuator dedicated to each individual resonator andindividually driven by the control device 270. The dedicated actuator isconfigured for strongly interacting with the respective associatedindividual resonator while interacting weakly with the remainingresonators. For example, as shown in FIG. 2, in case the tuningoperation relies on the thermo-optic technique (particularlyadvantageous in case of silicon waveguides) a micro-heater 251, 253 maybe thermally coupled to (e.g. placed above the microrings, e.g. over theSiO₂ upper cladding) each individual resonator 252, 254 respectively, sothat it is suitable to heat ideally only the respective associatedresonator (and possibly the straight bus waveguide 230 or 256 which theassociated resonator may be coupled to) while being ideally thermicallyisolated from the others.

FIG. 3 shows an alternative configuration of the optical filter 250. Forthe sake of clarity, it is assumed that the actuators are thermo-opticactuators (i.e. micro-heaters), but the present configuration equallyapplies to other tuning techniques. A massive tuning heater 310 isconfigured for heating substantially uniformly all the resonators 252,254 and 255 comprised within the optical filter 250. In addition, atrimming heater 320 is configured for selectively heating one or aplurality of resonators, but in any case it is configured forinteracting not uniformly with the totality of the resonators in theoptical filter 250. For example, the heater 320 may be placed inproximity (on the side or on top) of a single resonator 254 so as tointeract with this one single resonator more strongly than with theremaining resonators. In this way a differential temperature isgenerated which allows differential tuning of the resonators.

The optical components described in the present description, such as theoptical waveguides 230, 256 and the microrings 252, 254, 255 of FIG. 2and FIG. 3, may be fabricated by any fabrication process known in thefield of integrated optics, e.g. a layering process on a substrate, suchas an SOI wafer having a thickness of the buried oxide in the range of3-10 microns and a thickness of the top silicon in the range of 50-1000nm. The layering process may include the e-beam lithography and etchingsteps. A SiO₂ layer could be deposited as a top cladding.

In the following, an example of a method for optical filteringcomparative to the present invention will be described with reference toFIG. 4. This method, as well as those described with reference to FIGS.7A, B and C, may be implemented by operation of the scheme of theoptical device 200 of FIG. 2 or 3, described above, e.g. by tuning anoptical filter 250 comprising a plurality of series-coupled resonatorsin accordance with the present invention. Where useful for theunderstanding of the methods of the present description, reference willbe made to elements and corresponding reference numerals of FIGS. 2 and3, without restricting the scope of the method(s).

In FIG. 4 the horizontal and vertical axis of the Cartesian graphrepresent respectively time and optical frequency scale. Figures fromFIG. 4 to FIG. 8 show the case of an optical filter comprising two andno more than two series-coupled resonator. It is understood that theteaching of the present description equally applies to optical filterscomprising three or more series-coupled resonators.

For the sake of illustration it is assumed that the filter is tuned froman initially filtered optical channel (exemplary channel 2 representedby a dashed arrow) to a finally filtered optical channel (exemplarychannel 6), passing over the intermediate thru channels (in the examplechannels 3, 4 and 5 represented by solid arrows). It is also assumedthat the final channel has optical frequency (‘final frequency’) higherthan that of the initial channel (‘initial frequency’), even though theskilled reader would easily understand the more general case. Typicallyinitially and finally filtered channels are switched off during tuningoperation or they are let switched on but not used for communicationpurpose.

First (step not shown), a WDM optical signal comprising a plurality ofoptical channels having respective optical frequencies lying on a grid(‘WDM grid’) of allocated frequencies equally spaced by a givenfrequency spacing, said grid occupying an optical bandwidth, is receivedat the input port 257 of the optical filter 250.

It is noted that the WDM optical signal does not necessarily need tocomprise all the channels which may occupy said grid until it is filled.Actually, one or more of the allocated frequencies of the grid may bevacant. Nevertheless, the method and device of the present invention issuitable for processing a full-grid WDM signal and the examples belowwill refer to this case, without limiting the scope of the invention.

In step 1010, the initial channel is filtered by way of the opticalfilter 250. In the initial state 1010 the filter 250 is ‘enabled’ whichmeans that a respective optical resonance of each one of the pluralityof resonators falls within a given frequency band, typically comprisingthe center optical frequency allocated for the initial WDM channel(exemplarily the channel 2), said frequency band having a bandwidthsufficiently narrow to enable, independently from the single resonatorbandwidth, the filter to operate properly on the desired channel havingthe desired channel bandwidth (i.e. with suitable shape of the filterspectral responses as known in the art). Typically, the bandwidth isless than 15 GHz, preferably less than 10 GHz, and more preferably lessthan 5 GHz. This initial state may be achieved by properly tuning one ormore (e.g. all) of the individual resonators. In the present descriptionand claims, when reference is done to a position (in the frequencyspectrum) or to a distance of a resonance with respect to another one,reference is done to the peak of the resonance(s) of interest.

The resonances defined above are hereinafter called ‘initial resonancesof interest’ and their number equates the number of resonators withinthe filter. It is noted that typically a single resonator has aplurality of resonances characterized by their order and distributedalong the frequency spectrum, typically with constant periodicity. Thepresent invention equally holds independently by the specific order ofthe initial resonance of interest of each resonator. For practicalreasons, it is preferable that all the above initial resonances ofinterest of the resonators belong to the same order. In case all theindividual resonators are structurally identical (equal FSR) andthermo-optically actuated, this may be achieved, e.g., by setting allthe resonators at substantially the same mean temperature (‘meantemperature’ is the temperature averaged along the whole length of thering). In addition, it is noted that it is not strictly necessary thatall the resonators within the plurality of resonators have the same FSR.In the preferred case of the resonators having the same FSR, thecondition of step 1010 implies that each resonance of any resonator isaligned, within 15 GHz, with a respective resonance of any of the otherresonators.

Subsequently to step 1010 (e.g. because of the need of changing thechannel to be filtered) in step 1020 the optical filter 250 is subjectto a ‘disabling’ step, wherein the overall filtering function of thefilter is spoiled by introducing a certain separation between eachresonance of at least one of the plurality of the resonators fallingwithin the optical band of interest (WDM optical bandwidth) and therespective resonance of at least another different resonator which isthe resonance nearest (in the wavelength or frequency domain) torespectively said each resonance, the separation being greater than orequal to 25 GHz (see discussion below). In FIG. 4 solid curve 700represents the trajectory (in the time-frequency plane) of the initialresonance of interest of, e.g., said at least one resonator while solidcurve 710 represents the trajectory of the initial resonance of interestof said at least another different resonator. In the example of FIG. 4,only one resonance (i.e. the initial resonance of interest) of the atleast one resonator falls within the WDM band and the nearest resonanceof the at least another different resonator is the initial resonance ofinterest of the at least another resonator. In the example of FIG. 4,during disabling only the resonator 700 is tuned while resonator 710 iskept fixed. Curves 700′ and 710′ show a possible alternative to,respectively, curve 700 and 710 and curve 700″ and 710″ show anotherpossible alternative to, respectively, curve 700 and 710. Disabling ofthe filter continues at least until a separation equal to 25 GHz betweenthe two resonances is reached. However, the maximum separation reachedby the two resonances during the disabling step or in the entireprocedure of FIG. 4 may be, and typically is, higher. In FIG. 4 themaximum separation corresponding to curves 700 and 710 is exemplarilyone and half times the WDM channel spacing, while the one correspondingto curves 700′, 710′ and 700″, 710″ is respectively one and four timesthe channel spacing.

In order to illustrate the effects of step 1020, reference is done toFIGS. 5A and 5B which show thru (at port 258) and drop (at port 260)power responses of an optical drop filter 250 of the kind shown in FIG.2 or FIG. 3 and comprising two and no more than two microring resonators252, 254 series-coupled between the two bus-waveguides 230, 256. Themicrorings have the same ring radius equal to about 5 (±1%) μm and thusthe same corresponding FSR equal to about 2300±20 GHz. Silicon has beenselected as the core material of the waveguides constituting the opticalfilter 250, i.e. constituting both the resonators 252 and 254 and theoptical paths 230, 256. Preferably, the purity of silicon as the corematerial is higher than 90% in weight, more preferably higher than 99%.The doping level of the silicon core material is preferably below 10¹⁵defects/cm³. The choice of silicon is due to its high thermo-opticeffect which enables a large tuning range of the optical structures thusfabricated. For example, silicon as a core material allows for amicroring resonance tuning of at least 16 nm, preferably equal to about32 nm, with a relatively moderate range of the heater temperature, i.e.below 400° C. Silica may be used as a cladding material surrounding thesilicon waveguide core, e.g. in a buried or channel or ridge waveguideconfiguration. Alternatively other kind of materials could be used ascladding such as: polymers, spin on glass i.e. HSQ, Si3N4, etc. The highindex contrast waveguide obtained through the above material systemsallows fabricating microring resonators with very small radius andnegligible bending losses. Silicon waveguides height may suitably be inthe range of 100-300 nm and their thickness in the range of 200-600 nm.In the example described in FIG. 5, silicon waveguide cross section(both straight bus and microring) is about 488 nm wide and 220 nm high.The width of the section of the waveguide in proximity of the microringnarrows down to 400 nm wide. A SiO₂ top cladding with a refractive index(at a wavelength of 1550 nm and at a temperature of 25° C.) ofn_(clad)=1.446 has been included in the design. Silicon refractive indexhas been taken equal to 3.476 (wavelength of 1550 nm and temperature of25° C.). The calculated effective and group indexes of the Si waveguidewere respectively in the range of about 2.43-2.48 and 4.21-4.26. Thering to bus and ring to ring power coupling coefficients arerespectively 8.5% (±10%) and 0.24% (±10%), which may be exemplarilyobtained by a ring to bus gap equal to 132±10 nm and a ring to ring gapequal to 260±20 nm.

In calculating the optical responses, it has been assumed a realisticvalue for the total propagation loss of both the substantially straightsilicon waveguides (e.g. 230, 256) and of the microring waveguides 252,254 of the order of 3 dB/cm (comparable results are obtained in a rangefrom 2 to 5 dB/cm). In case of different values of microring losses, aproper choice of the bus-to-ring coupling coefficients may allowachieving the desired results in terms of hit losses.

A rigorous transfer matrix approach and a 3D Finite Difference TimeDomain (FDTD) approach have been respectively used for the calculationof the transfer functions and of the actual dimensional layout of theoptical components of the present description. Throughout the presentdescription, the TE polarization mode has been investigated, withoutrestricting the scope of the present invention. In particular, as regardpolarization, it is noted that some optical properties of the elements(or of their parts) of the present description, such as, e.g., theresonant optical frequencies, may depend on the specific polarizationmode of the optical field propagating therethrough. In the presentdescription, when reference is done to those optical properties, it isassumed a single polarization mode. Preferably the waveguidesconstituting those elements or their parts are apt to propagate only onepolarization (single polarization mode) or they are operated so as topropagate only one polarization (e.g. by exciting only one polarizationmode).

Curve 1110 (dotted) in FIGS. 5A and 5B represents the thru powerresponse (at thru port 258) when the optical filter is in the stateaccording to step 1010 above, i.e. ‘enabled’ and tuned on a givenchannel (the initial channel), illustrated by the dashed arrow atconventionally zero frequency. In particular, the two respective initialresonances of interest of the two microring resonators are tuned so asto be substantially aligned, i.e. to fall within a frequency bandcentered on the initial channel and having bandwidth less than or equalto 10 GHz. Curve 1120 (dotted) in FIG. 5A represents the drop powerresponse (at drop port 260) corresponding to the thru response 1110. Thesolid arrows represent the channels neighboring the initial channel,with an exemplary spacing of 200 GHz.

Curves 1130 (dashed), 1140 (continuous) and 1150 (dash-dotted) representthe drop response at three instants (e.g. states A, B and C with regardto curves 700 and 710 in FIG. 4) of an exemplary realization of step1020 (either in a final or an intermediate state), wherein theseparation between the respective optical resonances of the first andsecond resonator is respectively 100 GHz, 200 GHz and 300 GHz. Curves1160 (dashed), 1170 (continuous) and 1180 (dash-dotted) in FIG. 5Brepresent the thru response corresponding to the drop responses 1130,1140 and 1150 respectively.

FIGS. 5A and 5B show how the overall resonance and filtering function ofthe optical filter 250 is spoiled by mutually separating the initialresonances of interest of the first and second resonator (filter‘disabling’). In particular, the disablement shown in FIGS. 5A and 5B isobtained by leaving the resonance of the resonator coupled closest tothe first optical path 230 (i.e. the input-to-thru waveguide)unperturbed, i.e. in correspondence to the initial channel, and bytuning only the resonator coupled closest to the drop waveguide 256 soas to move its respective resonance away from the resonance of the otherresonator.

Once the filter is disabled, i.e. the separation is greater than 25 GHz(see discussion below), it is adapted to be preferably massively tuned(optional step 1030) over the WDM band without affecting or weaklyaffecting the WDM channels ‘crossed’ by any resonance of any individualresonator of the optical filter. The expression ‘massive tuning’ meansthat all the resonances of the resonators are moved in the frequencydomain by a respective frequency interval greater than the WDM frequencyspacing, while maintaining a distance between the resonances separatedaccording to the above greater than or equal to the separation achievedduring the disabling step (which in turn is greater than or equal to 50GHz).

Preferably, during massive tuning all the resonators of the filter aretuned substantially in unison (uniformly and contemporarily), i.e. theoverall response functions of the optical filter rigidly move in thefrequency domain. Exemplarily, in FIG. 4 and curves 700 and 710, themassive tuning is performed ‘rigidly’ and it ends when the resonance ofone of the individual resonators is in the proximity of the centralallocated WDM frequency of the final channel.

The effect of the disablement during massive tuning is derivable fromFIG. 5B: the maximum power loss hit at 100 GHz, 200 GHz and 300 GHzresonance separations is respectively about 1 dB, 0.6 dB and 0.5 dB onthe thru channels (channels 3, 4 and 5 of FIG. 4) ‘crossed’ by theresonance of interest of the ring coupled closest to the input-to-thruwaveguide 230 while being tuned (assuming ring propagation loss of about3 dB/cm). Such resonance of interest in FIG. 5B is represented at zerofrequency. It's worth to note that the drop response (FIG. 5A) is notparticularly significant during massive tuning, since typically duringmassive tuning operations the output from the drop port is neglected.

FIGS. 6A, 6B and 6C show the thru dispersion response (at thru port 258)corresponding respectively to the thru power responses 1160, 1170 and1180 of FIG. 5B (and corresponding respectively to a resonanceseparation of about 100 GHz, 200 GHz and 300 GHz).

FIG. 5C shows on the horizontal axis the mutual distance (in absolutevalue) between the closest resonances of a two-ring optical filter andon the vertical axis the corresponding power loss at a thru opticalfrequency overlapping the resonance of either the ring closest to theinput-to-thru waveguide (dashed curve 1190) or the ring adjacentlycoupled to the previous ring (continuous curve 1192). In case of a dropfilter, the latter ring is the one adjacently coupled to the dropwaveguide (e.g. waveguide 256 of FIG. 2 and FIG. 3).

FIG. 5D correspondingly shows on the horizontal axis the mutual distance(in absolute value) between the closest resonances of a two-ring opticalfilter and on the vertical axis the corresponding dispersion hit at athru optical frequency overlapping the resonance of either the ringclosest to the input-to-thru waveguide (dashed curve 1194) or the ringadjacently coupled to the previous ring (continuous curve 1196).

The following table 1 shows the corresponding numerical values, whereinthe second column corresponds to curve 1190, the third one to curve1194, the fourth column to the curve 1192 and the last column to curve1196.

TABLE 1 Separation Loss ring #1 Disp ring #1 Loss ring #2 Disp ring #2(GHz) (dB) (ps/nm) (dB) (ps/nm) 25 4.9 150 4.3 50 50 2.1 118 1.6 18 751.3 108 0.75 10.5 100 1 105 0.4 6 125 0.8 103 0.3 4 150 0.72 102 0.2 2.5175 0.66 101.5 0.15 2 200 0.61 101.2 0.12 1.5 300 0.55 100.5 0.06 0.65400 0.51 100 0.035 0.4 500 0.5 100 0.02 0.26 600 0.5 100 0.015 0.2 8000.5 100 0.01 0.15

In general, Table 1, FIGS. 5C and 5D show that when the microringresonances are mutually separated by an amount greater than 25 GHz (i.e.greater than 0.2 nm) the extra loss and the extra dispersion (which areboth mainly given by the ring adjacently coupled to the input-to-thruwaveguide) on any channel during massive tuning of the filter are lowerthan or equal to respectively about 5 dB and about 150 ps/nm.Preferably, the separation between the two resonances is greater than orequal to 50 GHz, so as to obtain a loss and dispersion hit less thanrespectively 3 dB and 150 ps/nm. The above values of the loss anddispersion hit depend on the structure, materials and parametersexemplarily used above.

The technical specifications which according to the Applicant arepreferable for a hit-less tunable OADM filter, are simultaneously thefollowing:

-   1. the extra loss (loss hit) suffered by any of the thru channels    during filter massive tuning less than or equal to the loss    uniformity requirement, i.e. less than or equal to about 0.5 dB.-   2. the extra dispersion (dispersion hit in absolute value) induced    on the thru channels during filter massive tuning lower than about    150 ps/nm (better 100 ps/nm).

It is noted that although the specification on the maximum extradispersion of the thru channels for a generic tunable OADM is commonly+/−20 ps/nm (being that it may be possible to cascade up to 16 OADMS ina telecommunication link while maintaining the accumulated dispersionbelow about 320 ps/nm), nevertheless during the transient time of thetuning procedure (i.e. over some tenths of milliseconds) an extradispersion up to 100-150 ps/nm can be tolerated without significantlyaffecting the transmission performances.

The Applicant has found that when the detuning is less than about 125GHz (1 nm), the maximum loss at a frequency matching the resonance ofthe ring adjacently coupled to the input-to-thru waveguide is greaterthan about 0.8 dB and the dispersion larger than about 105 ps/nm. TheApplicant has also found that it is possible to mitigate such large hitsby increasing the mutual resonance distance, as becomes now clear fromTable 1 and FIGS. 5C and 5D. Accordingly, the Applicant has found thatit is preferable to maintain the relative distance between therespective resonances of a first and a second resonator within theoptical filter of the invention during massive tuning of the filter at avalue greater than or equal to 125 GHz, so as to obtain a maximum losson the thru channels less than or equal to 0.8 dB. More preferably, suchresonance distance is greater than or equal to 150 GHz, in order tomitigate the hit at a value less than or equal to 0.7 dB. Even morepreferably, the resonance distance is greater than or equal to 200 GHz,corresponding to a hit no more than 0.6 dB. Further more preferably,when the separation is greater than or equal to 300 GHz (2.4 nm in thenear infrared band), the extra loss on thru channels during massivetuning of the filter is lower than or equal to about 0.5 dB and theextra dispersion equal to about 100 ps/nm.

In addition to the above, the Applicant has found that while increasingthe mutual distance of the resonances using the thermo-optic effect, atrade-off exists between the consequent decrease of power and dispersionhit and the increase of the thermal cross talk. In fact, the differencein the resonance position corresponds to a difference in the thermalstate of the at least two rings of interest, i.e. a difference in thering mean temperature. For a given resonance separation, a correspondingdifference of the ring mean temperatures exists, which depends on thestructure and material of the rings. For example, for a two-ring siliconfilter as described above, a resonance separation of about 800 GHz and400 GHz corresponds to a difference in the ring mean temperature ofrespectively about 80° C. and about 40° C., roughly speaking. In turn,for a given difference in the ring mean temperature a correspondingthermal cross-talk exists, i.e. a certain amount of thermal energy flowsfrom the hotter ring to the cooler one and/or from the heater heatingthe respective ring to the other (unwanted) ring. Again, the thermalcross-talk will depend on the choice of materials and structures of therings and their coupling region, on the thermal isolation among the tworings and on the structure, layout and thermal coupling of therespective heaters. In general, for a given target difference in thering mean temperature to be maintained, the higher is the thermalcross-talk, the higher is the difference in the heat radiated by the twoheaters associated to the two rings and consequently the higher is thetotal power consumption. Since typically the higher is the difference inthe heat radiated by the two heaters the higher is also the differencein the temperatures of the two heaters, this results also in a higherthermal wear and tear.

Accordingly, the Applicant has found that it is advantageous to keep theresonance separation less than or equal to about 800 GHz, morepreferably less than or equal to about 600 GHz, even more preferablyless than or equal to about 500 GHz. These maximum values are consistentwith the fact that, as now clear from Table 1, FIGS. 5C and 5D, the lossand dispersion hits asymptotically tend to fixed values (exemplarilyabout 0.5 dB loss and 100 ps/nm dispersion hits).

Referring now back to FIG. 4, in step 1040 the filter is enabled again,so that, once enabled (state 1050), a respective optical resonance ofeach one of the plurality of resonators falls within a frequency band,having the bandwidth described above for the initial frequency band, andtypically comprising the center optical frequency allocated for thefinal WDM channel (exemplarily channel 6).

The resonances defined above are hereinafter called ‘final resonances ofinterest’ and their number equates the initial resonances of interest.While in FIG. 4 the initial and final channels are filtered by way ofthe same resonances of the two rings translated in frequency (i.e. thefinal resonances of interest have the same order of the initialresonances of interest), in another embodiment the order of the finalresonances of interest, at least of one or more of the plurality ofresonators, may be different from that of the initial resonances ofinterest.

The ‘enabling’ step 1040 may be performed by replicating back the samesteps followed for filter disabling 1020 with the role of the two ringsof interest mutually exchanged, as shown in FIG. 4. With reference toFIG. 4, exemplarily the resonance of said at least one resonator (curve700 or 700′ or 700″) is maintained at the target frequency while theresonance of said at least another different resonator (curve 710 or710′ or 710″) is moved toward the target frequency, so as to passthrough states C′, B′ and A′ respectively corresponding to states C, Band A.

The specific starting and ending points of the dynamic steps 1020, 1030and 1040 shown in FIG. 4 (and the following FIG. 7) are purelyconventional and for illustrative purpose only. This is particularlytrue when determining the boundaries between filter disabling and filtertuning and between filter tuning and filter enabling. Conventionally,the ending point of the disabling step (which coincides with thestarting point of the tuning step) may be taken at the instant when theseparation between the resonances of interest reaches a givenpredetermined value, which in any case needs to be not less than 25 GHz.Similarly, the starting point of the enabling step may be taken at theinstant when the separation between the resonances of interest goesbelow a further given predetermined value, which may be equal to thepredetermined value above or different, but in any case not less than 25GHz. Exemplarily, in FIG. 4 the end of disablement and the start ofenablement both are conventionally taken at one and half times the WDMchannel spacing for curves 700 and 710, while for curves 700′, 710′and700″, 710″ they respectively are at one and (roughly) two times thechannel spacing. However, the position in time of the end and startpoints above changes while changing the above predetermined value(s).For example, in correspondence to dotted lines 700″ and 710″, if thepredetermined value is taken equal to the maximum separation of theresonances (four times the spacing), then the disabling step may beconsidered ending at the maximum separation (vertical dashed line 1000),which may also be considered as the starting point of the successiveenabling step 1040, thus making the ‘tuning’ step 1030 to collapse andeventually vanish. Dashed line 1000 may be conventionally taken as theseparation point between enabling and disabling steps also for curves700, 710 and 700′, 710′. In this sense the massive tuning step 1030 maybe considered optional.

In the example shown in FIG. 4 the resonance of interest of both the atleast one resonator (curve 700) and the at least another resonator(curve 710) ‘hits’ successively the thru channel numbered 3, 4, and 5,for a total number of six hits. It is noted that not all the six hitsshown in FIG. 4 are equal in magnitude, especially with regard tooptical power. The Applicant has noted that in the comparative methodillustrated in FIG. 4, the power hits occurring at point B and B′ forcurves 700 and 710 of FIG. 4 are different from those occurring duringthe massive tuning step 1030, since the relative distance between theresonances is smaller in points B and B′ (e.g. one channel spacing) thanduring massive tuning (e.g. 1.5 times the channel spacing). In addition,as now clear from FIGS. 5A, B, C and D which refer to a two-ring filter,for a given relative distance between the resonances of interest of thetwo resonators, the biggest power and dispersion hit occurs when thefrequency of a thru channel equates the resonance of the resonatorcoupled closest (adjacently) to the input-to-thru waveguide (curve 1190and 1194 of FIGS. 5C and 5D). Thus the hit is worse in state B or instate B′ depending on curve 700 representing respectively the resonatorcoupled closest to the input-to-thru waveguide (waveguide 230) or theother resonator. For example, assuming a channel spacing equal to 100GHz and assuming that curve 700 represents the ring series-coupled tothe ring adjacently coupled to the input-to-thru waveguide, then thepower hit on channel 3 in the state B is equal to about 0.4 dB (seeTable 1 and FIG. 5C) and the subsequent hits on channel 4 and 5 is equalto about 0.2 dB. As regard to curve 710 (ring closest to theinput-to-thru waveguide) the hits during massive tuning on channel 3 and4 is equal to about 0.7 dB, while the hit in the state B′ on channel 5is about 1 dB.

Based on the discussion above, the Applicant has found that it isadvantageous to avoid that any resonance of any resonator hits a thruchannel when the relative distance of such resonance with respect to therespective closest resonance of any other resonator is less than themaximum resonance separation set for filter tuning (in FIG. 4 the hitsto be avoided correspond to state B and B′ for curve 700 and 710 and thehits on channel 3 and channel 5 of curve 700″ and 710″). In particular,the latter sentence is true in case the maximum resonance separation isless than or equal to two times the channel spacing. In case the maximumresonance separation is larger than two times the channel spacing, theApplicant has found a solution that avoids any hit in correspondence toa resonance distance smaller than two times the channel spacing.

Moreover, the Applicant has found that it is particularly advantageousto avoid that any resonance of the resonator coupled closest to theinput-to-thru waveguide hits a thru channel when the relative distanceof such resonance with respect to the closest resonance of any otherresonator is equal to or less than the channel spacing.

The Applicant has found a solution to the problem above, the variousembodiments of which are shown in FIGS. 7A to C, wherein the samereference signs of FIG. 4 have been used for the same features, whereapplicable, and the same exemplary assumptions made for FIG. 4 are used.In general, the solution envisages that during enabling and/or duringdisabling step, at least one among the initial resonances of interest(and/or respectively the final resonances of interest) is moved in thefrequency spectrum outside the region of the frequency spectrumcomprised between the initial channel and the final channel (‘resonanceovershooting’). Preferably, said at least one initial and/or finalresonance of interest belongs to a resonator different from the onecoupled closest to the input-to-thru waveguide. The method isparticularly suitable to change the filtering from an initial channel toa final channel within a plurality of WDM channels, while leaving thethru channels with a minimum alteration or no alteration at all.

FIG. 7A shows a possible embodiment of the tuning technique inaccordance to the present invention. The reference sign 700A refers tothree possible alternative trajectories (dotted, solid and dashedcurves) of the resonance of interest of one of the two resonators andthe reference sign 710A refers to three possible alternativetrajectories of the resonance of interest of the other of the tworesonators. In particular, dotted lines show an exemplary variant of thetrajectory pattern. Independently from the symbol used (respectivelydotted, solid and dashed), any combination obtained by a choice of oneout of the three curves 700A and one out of the three curves 710A issuitable to the invention. A choice in accordance with the same symbol(respectively dotted, solid and dashed) gives an exemplary maximumresonance separation (during massive tuning) equal to 1.5 times thechannel spacing. Curves 720A and 730A (dot-dashed) show further possiblealternative paths of the resonance curves wherein the massive filtertuning step 1030 may be considered absent. The main difference withrespect to the method described with reference to FIG. 4 is that nowduring the step 1020 of filter disabling and the step 1040 of filterenabling, one of the two resonators is tuned so that the respectiverelevant resonance moves in the frequency spectrum staying on oppositesides with respect to, respectively, the initial frequency and the finalone. As a consequence, any hit before the maximum resonance separationis reached (e.g. any hit during the enabling and the disabling steps)can be avoided, as long as the maximum resonance distance does notexceed the value of twice the channel frequency spacing. In FIG. 7A thehits occur solely during massive tuning step, in a total number of sixin correspondence to the dotted and solid curves, which increases up toeight hits (hitting also channel 1 and 7) choosing both the dashedcurves. Moreover, in case the maximum separation does exceed the doubleof the frequency spacing, the present solution allows avoiding any hitin correspondence to a resonance separation less than twice thefrequency spacing. Due to the point symmetrical configuration of thepatterns shown in FIG. 7A, the overall power hit of the entire procedureis not affected by the choice of which one of the two rings (the onecoupled closest to the input-to-thru waveguide or the other) correspondsto curve 700A or 710A. In particular, the resonator whose resonance ofinterest overshoots during disabling of the filter is different from theresonator whose resonance of interest overshoots during enabling of thefilter.

FIG. 7B shows an alternative embodiment of the tuning technique inaccordance to an embodiment of the present invention. While thedisabling step is similar to that described in FIG. 4, the enabling stepis now performed in accordance to the technique shown in FIG. 7A. Due tothe asymmetry of the pattern of FIG. 7B, it is important which ringcorresponds to which curve. In accordance to a preferred embodiment, itis a ring different from the one closest to the input-to-thru waveguidewhich overshoots with respect to the strict tuning range which startsfrom the initial frequency and ends at the final one. Curve 700Bcorresponds to a ring different from the one closest to theinput-to-thru waveguide while curve 710B corresponds to the ring closestto the input-to-thru waveguide. Curves 700B′ and 710B′ show possiblealternative paths of the resonance curves 700B and 710B, respectively.Curves 720B and 730B show further possible alternative paths of theresonance curves, respectively, wherein the massive filter tuning step1030 may be conventionally assumed as shown in the figure or it may beconsidered absent. The patterns used for the enabling and disablingsteps may be mutually exchanged provided that care is taken to mutuallyexchange also the roles of the two rings. The main difference withrespect to FIG. 7A is that now a hit is tolerated at a resonanceseparation less than two times the channel spacing (exemplarily equal tothe channel spacing during filter disabling), provided that care istaken that this hit is caused by a ring distal with respect to theinput-to-thru waveguide. For example, assuming a channel spacing equalto 100 GHz, the power hit due to the ring distal from the input-to-thruwaveguide is equal to about 0.4 dB on channel 3 (state B of FIG. 7B-seeFIG. 5C) and about 0.2 dB on channel 4 and 5. As regard to curve 710B(assumed to represent the ring closest to the input-to-thru waveguide)the hit during massive tuning on channel 3, 4 and 5 is equal to about0.7 dB, including the hit in the state C′ on channel 5. An advantage ofthe present embodiment is that the maximum frequency tuning rangespanned by both the rings is lower than those shown in FIG. 7A(respectively 4.5 times the channel spacing—for the distal ring—and from5 to 6 times).

FIG. 7C shows two further alternative embodiments of the tuningtechnique in accordance to the present invention. Curves 700C and 720Crefer to the resonator closest to the input-to-thru waveguide and curves710C and 730C are the corresponding trajectories respectively associatedto the other resonator. Also in this case the patterns are asymmetric,with the same implications described above with reference to FIG. 7B.The main difference is that now, while the resonance of the distalresonator overshoots (exemplarily in the disabling step), the proximalresonator may remain unperturbed (curve 700C) or it may slightly‘overshoot’ in the same direction of the distal resonator (curve 720C).Again, the pattern allows that the hit in state B and B′ is caused bythe distal ring and it is thus mitigated.

The present invention further contemplates any combination of thepatterns shown in FIG. 7A to 7C. It is noted that in all embodiments ofFIGS. 7A-C, either during enabling or during disabling the resonance ofthe ring distal from the input-to-thru waveguide is preferably moved inthe opposite direction with respect to that needed for going from theinitial frequency to the final one.

FIG. 8 illustratively shows the effects of the solution in accordance tothe present invention, with the assumption that the channel spacing isequal to 100 GHz. Curve 1410 represents either state B or state B′ ofFIG. 7A (dotted lines) and curve 1420 correspondingly representsrespectively either state C or state C′ of FIG. 7A (dotted lines), withthe ‘proximal’ ring overshoot (assuming that the massive tuning takesplace in the positive frequency region). The thru channel +100 GHz awayfrom the channel at zero frequency suffers a hit only in correspondenceto a resonance separation of about one and half the channel spacing, incontrast to a possible hit in correspondence to only one channel spacingin absence of the overshoot.

Although the present invention has been disclosed and described by wayof some embodiments, it is apparent to those skilled in the art thatseveral modifications to the described embodiments, as well as otherembodiments of the present invention are possible without departing fromthe essential features thereof/the scope thereof as defined in theappended claims.

The invention claimed is:
 1. A method for filtering an optical signalcomprising a plurality of channels lying on a grid of opticalfrequencies equally spaced by a frequency spacing and occupying anoptical bandwidth, the method comprising: operating an optical filtercomprising resonators, each resonator having a respective free spectralrange, wherein a first resonator is optically coupled to the opticalsignal and the remaining one or more resonators are optically coupled inseries to the first resonator, so that a respective resonance of eachone of the resonators falls within a first frequency band havingbandwidth less than or equal to 15 GHz; first tuning at least oneresonator with respect to at least another resonator to obtain aseparation between a resonance of the at least one resonator and aresonance of the at least another resonator greater than or equal to 25GHz; and second tuning the at least one resonator and the at leastanother resonator such that the respective resonance of each one of theresonators falls within a second frequency band, different from thefirst frequency band, having bandwidth less than or equal to 15 GHz;wherein during a period between and including the first tuning and thesecond tuning, at least one among the respective resonance of the atleast one resonator and the respective resonance of the at least anotherresonator is tuned outside a frequency region spanning between, andincluding, the first and the second frequency band.
 2. The method ofclaim 1, wherein the at least one among the respective resonance of theat least one resonator and the respective resonance of the at leastanother resonator belongs to a resonator different from the firstresonator.
 3. The method of claim 1, wherein both the respectiveresonance of the at least one resonator and the respective resonance ofthe at least another resonator are also moved outside the frequencyregion comprised between, and including, the first and the secondfrequency band.
 4. The method of claim 3, wherein one among therespective resonance of the at least one resonator and the respectiveresonance of the at least another resonator is moved outside thefrequency region either during the first tuning and the second tuningand the other among the respective resonance of the at least oneresonator and the respective resonance of the at least another resonatoris moved outside the frequency region respectively either during thesecond tuning and the first tuning.
 5. The method of claim 1, furthercomprising, after the second tuning: third tuning all the resonators ofthe optical filter so as to tune all respective resonances of theresonators by a frequency interval greater than the frequency spacingwhile maintaining a distance between the resonance of the at least oneresonator with respect to the respective resonance of the at leastanother resonator not less than the separation.
 6. The method of claim5, wherein during the third tuning, all the resonators are tunedsubstantially in unison so as to equally and contemporarily move all therespective resonances of the resonators.
 7. The method of claim 1,wherein the at least one among the respective resonance of the at leastone resonator and the respective resonance of the at least anotherresonator is moved outside the frequency region within less than onechannel spacing from a proximal edge of the frequency region.
 8. Themethod of claim 1, wherein all the resonators of the optical filter havethe same free spectral range.
 9. The method of claim 1, wherein theseparation is greater than or equal to 150 GHz.
 10. The method of claim1, wherein the separation is less than or equal to 1000 GHz.
 11. Themethod of claim 1, wherein the resonators of the optical filter areoptically coupled in series between a first optical path propagating theoptical signal and a second optical path.
 12. The method of claim 1,wherein the resonators of the optical filter comprise two and no morethan two resonators.
 13. The method of claim 1, wherein during the firsttuning, the optical filter obtains the separation between each resonanceof the at least one resonator falling within the optical bandwidth withrespect to a respective closest resonance of the at least anotherresonator.
 14. The method of claim 1, wherein a first channel belongingto the optical signal at least partially overlaps the first frequencyband and a second channel belonging to the optical signal at leastpartially overlaps the second frequency band.
 15. The method of claimclaim 1, wherein the at least one among the respective resonance of theat least one resonator and the respective resonance of the at leastanother resonator belongs to a resonator different from the firstresonator.
 16. The method of claim 2, wherein both the respectiveresonance of the at least one resonator and the respective resonance ofthe at least another resonator are also moved outside the frequencyregion comprised between, and including, the first and the secondfrequency band.
 17. The method of claim 2, further comprising, after thefirst tuning and before the second tuning: intermediate tuning all theresonators of the optical filter so as to move all respective resonancesof the resonators by a frequency interval greater than the frequencyspacing while maintaining a distance between the resonance of the atleast one resonator with respect to the respective resonance of the atleast another resonator not less than the separation.
 18. An opticaldevice comprising: an optical filter having: an input port for receivingan optical signal comprising a plurality of channels lying on a grid ofoptical frequencies equally spaced by a frequency spacing and occupyingan optical bandwidth, and an output port; a first optical path opticallyconnecting the input port to the output port; and resonators, eachhaving a respective free spectral range, wherein a first resonator isoptically coupled to the first optical path and the remaining one ormore resonators are optically coupled in series to the first resonator;and a control system operatively connected to the resonators of theoptical filter, the control system being configured to: operate theoptical filter so that a respective resonance of each one of theresonators falls within a first frequency band having bandwidth lessthan or equal to 15 GHz; first tune at least one resonator with respectto at least another resonator to obtain a separation between a resonanceof the at least one resonator and a resonance of the at least anotherresonator greater than or equal to 25 GHz; and second tune the at leastone resonator and the at least another resonator such that therespective resonance of each one of the resonators falls within a secondfrequency band, different from the first frequency band, havingbandwidth less than or equal to 15 GHz; wherein during a period betweenand including when the control system first tunes and second tunes, thecontrol system tunes at least one among the respective resonance of theat least one resonator and the respective resonance of the at leastanother resonator outside the frequency region between, and including,the first and the second frequency band.
 19. The device of claim 18,wherein the resonators of the optical filter are optically coupled inseries between the first optical path and a second optical path.
 20. Anoptical communication system comprising: a transmitter, a receiver, anoptical line optically connecting the transmitter and the receiver; andan optical device coupled along the optical line, the optical devicecomprising: an optical filter having: an input port for receiving anoptical signal comprising a plurality of channels lying on a grid ofoptical frequencies equally spaced by a frequency spacing and occupyingan optical bandwidth, and an output port; a first optical path opticallyconnecting the input port to the output port; and resonators, eachresonator having a respective free spectral range, wherein a firstresonator is optically coupled to the first optical path and theremaining one or more resonators are optically coupled in series to thefirst resonator; and a control system operatively connected to theplurality of resonators of the optical filter, the control systemconfigured to: operate the optical filter so that a respective resonanceof each one of resonators falls within a first frequency band havingbandwidth less than or equal to 15 GHz; first tune at least oneresonator with respect to at least another resonator to obtain aseparation between a resonance of the at least one resonator and aresonance of the at least another resonator greater than or equal to 25GHz; and second tune the at least one resonator and the at least anotherresonator such that the respective resonance of each one of theresonators falls within a second frequency band, different from thefirst frequency band, having bandwidth less than or equal to 15 GHz;wherein during a period between and including when the control systemfirst tunes and second tunes, the control system tunes at least oneamong the respective resonance of the at least one resonator and therespective resonance of the at least another resonator outside thefrequency region comprised between, and including, the first and thesecond frequency band.